AND, yes the trailer brakes should stop the trailer but in reality the truck may be minimal because of the issue GM had at the time. You may not notice it much when daily driving but when put under more stress it easily shows because 100 % of the trailer is not on the trailer brakes. My FW in no way covers 100% of the FW braking needs even when adjusted as good as possible. People often get far better braking after installing disc brake kits on the rig.
It's unlikely most towables have the braking capacity of our vehicles. Yes the breakaways do work but in brake performance measurements I would bet that they are on the weak side and dont compare to the stopping distances of our vehicles. So often times some of the braking may also be on the vehicle. If your trailer brakes are not doing as much as they absolutely can than it may show quickly on that truck with the issues it may have.